The present invention relates generally to a system and technique for steering the direction of propagation of a beam of electromagnetic energy, and more particularly to a system and method for increasing the range of off-axis pointing of a composite laser beam emanating from a phased array of optical telescopes.
The concept of optically phasing an array of laser telescopes in a manner to achieve the most optical power on-axis at a distant stationary object is presently known. Apparatus and a technique for implementing such a concept are disclosed, for example, in U.S. Pat. No. 4,639,586, issued to Janet S. Fender et al on Jan. 27, 1987, which patent is incorporated herein by reference.
The phasing of the telescopes in the Fender et al patent is accomplished by sampling their outgoing wavefronts, and generating correction signals therefrom that are applied to correction mirrors via mirror control mechanisms. The mirror control mechanisms effect a fine dynamic control of the optical path lengths through the telescopes to provide a zero optical path difference condition.
The aforementioned Fender et al patent also discloses an image processor system and an associated algorithm for generating the required correction mirror control signals. This image processor system and algorithm are disclosed in greater detail in U.S. Pat. No. 4,689,758 issued to Richard A. Carreras on Aug. 25, 1987. The Carreras patent is also incorporated herein by reference.
The phased multiple telescope array systems and techniques disclosed in the Fender et al and Carreras patents have been developed and optimized to achieve the most optical power on-axis and on a stationary target. Therefore a figure of merit of the systems is the Point Spread Function which appears on a target plane, optimized for maximum intensity on-axis.
It is desirable in certain applications to have a phased multiple telescope array system in which the Point Spread Function can be steered to follow a moving target and thereby continuously deposit the maximum optical intensity on the target. The invention disclosed herein arose from an investigation of the problem of how to move the Point Spread Function over a large off-axis angle while still maintaining a phased outgoing wavefront.
To some degree, the small movable correction mirrors of the phased telescope system disclosed in the Fender et al patent can be used to steer, point or otherwise move the composite beam as it leaves the telescopes. This is a very desirable feature, since moving small steering mirrors is much easier than moving an entire telescope platform. An additional advantage of using such small mirrors is that they can be moved much faster and positioned much more accurately. However, because the range of the mirror dynamic control mechanisms is small, such systems have heretofore been unsuitable for moving target applications.